It's not entirely clear to me what they could do about it. Since they are an email provider, they probably don't have control over the networks their customers send email from, and what else their customers do with their domains.

Someone could decide to forward their other mail to their fastmail account. Should they then potentially risk email their other customers send to that address? DMARC headers tries to solve this, but the world is dirty, mailing list software suck, and their they would have to take the blame for problems outside their control.

I can understand the decision. They could probably do something to show good intentions, like flagging suspicious email and making sure their own email software shows appropriate warnings, but it's never going to be perfect.